I recently acquired this tiny unposted cover while clearing my in-law's house for sale. It's the birth announcement for my mother-in-law. Prompted me to wonder what was the smallest acceptable cover, and what was the smallest standard stationery cover in the past. So I was pleased to find this topical thread.
This item measures 2 5/8 in. H x 3 5/8 in. W.
I can see that it is not the smallest ever, nor did I expect it to be. But I do wonder if this was the smallest standard size stationery, at least in the 1930s US.... It would seem the proud parents had perhaps 100 of these worked up and mailed out in a farming community in upstate New York, so it must have been a standard size -- perhaps relatively common -- for printers and stationers at the time.
Anyway, it's the smallest in my accumulation. The younger generation got a kick out of the precision of the denomination.